1. Field of the Invention
This invention is directed to an improved hinge assembly for a vacuum packaging machine. More particularly, the inventive hinge assembly is equipped with a movable hinge block with an arcuate slot therein and a stationary hinge plate with pins extending outward therefrom which collectively result in a virtual pivot point for a lid which is outboard of the machine. The hinge assembly allows increased access to the machine by pivoting the lid to a greater angle and also creates an increased moment arm so that smaller lid lifting springs can be used.
2. Description of the Related Art
Packaging within a vacuum can add significantly to the shelf life of certain foods such as meats, dairy products, etc. Vacuum packaging machines designed for packaging of such foodstuffs within an evacuated chamber have been in use for many years. Typically such machines have a stainless steel tub within which is positioned a heat sealing apparatus. The tub is equipped with a hinged lid with perimeter seals for sealing the top of the tub when the lid is closed. A vacuum pump is connected to the tub. Food to be packaged is put in a heat sealable wrap and the wrapped food is positioned in the tub. The lid is closed by an operator and a vacuum is drawn on the tub, temporarily keeping the lid closed and sealed. The heat sealing apparatus is then operated to seal the package, thus yielding an air evacuated, sealed food package, whereupon the vacuum seal is released and the lid automatically raises due to the lifting action of gas springs or the like.
In order for the lids to properly seal the vacuum tubs and withstand the considerable forces exerted by the vacuum, they are generally quite heavy. Vacuum forces within a typical machine can approach 15 lbs. per square inch. This means that a typical lid made of clear Lexan must be a full one inch thick, weighing upwards of 40 pounds. Prior art vacuum packaging machines have used conventional hinges on these lids with pivot points near and inboard of the rear of the tub. In order to provide uninterrupted access to the front and sides of the machine to load and unload food packages, no lifting mechanism can be placed in these areas. This means that the lifting mechanisms, e.g. gas springs, for lifting the lid must be positioned closely adjacent to the hinges at the rear of the tub, thus limiting the effective gas spring lifting moment arm to a length of about one inch. This requirement also limits the placement points for the gas springs to a single position, thus virtually eliminating any ability to adjust spring leverage. With a forty pound lid and just a one inch moment arm, prior art machines have needed to use dual lifting gas springs, each with capacities of 150 lbs or more. Constant opening and closing of the heavy lids against the force of these large capacity springs places considerable stress on the machine components, including the lid connection to the hinges, the connections between the springs and the hinges and the hinge components themselves. Furthermore, these machines are usually used in refrigerated environments, which reduces the effectiveness of even new gas springs by 5% or more. In a machine which requires 150 pound capacity springs for effective lid operation, this percentage reduction can have a serious impact on the speed and efficiency of lid operation, thus reducing the potential throughput of food packages.
In addition, on prior art machines, the placement of the hinge pivot point inboard of the rear of the machine necessitates that the rear lip of the stainless steel tub be radiused to accommodate the lid when it opens. Manufacturing production of a tub with a radiused rear edge is considerably more expensive than a tub with just square surfaces.
It is clear then, that a need exists for a vacuum packaging machine with an improved hinge assembly. Such a hinge assembly should preferably be capable of operation by smaller capacity gas springs, should allow adjustment of spring attachment position, and should be compatible with vacuum tubs with squared rear edges. In addition, the improved hinge assembly should reduce the stresses placed on component parts by the lifting mechanism and should be relatively simple and economical to manufacture.